The invention pertains to automatic exclusion circuitry for establishing line privacy in a multistation key telephone system.
In telephone systems having two or more phone stations capable of being connected to a common telephone line, it is many times desirable to provide line privacy for a person using the telephone line at a first of the stations by preventing any remaining stations from picking up the line after a telephonic communication has been initially established at the first station. To provide such privacy, various circuits and switching devices have been developed for excluding subsequent connections to a busy line, and these circuits and devices are commonly referred to as exclusion circuits or exclusion switches.
In some cases, the exclusion is effected by manually operating an exclusion switch mounted on or adjacent the telephone set. Preferably, however, the privacy feature is provided by an automatic exclusion circuit in which the users of the telephone system need not remember to throw a switch to activate the exclusion mode, and need not remember to return the switch to the nonexclusion position upon terminating the phone call.
Accordingly, it is an object of the present invention to provide an automatic exclusion circuit for a key telephone system of the type in which the telephone set at each of the plurality of stations is normally disconnected from the common telephone line and is selectively connected to the line by a line switching device such as a relay that in turn is controlled by a latching circuit operating in response to a manually actuated line selector switch and a line condition sensing circuit.
A related object is to provide such an exclusion circuit for a key telephone system of the above type and wherein the system is further characterized in that each station has a separate line switching device, latching circuit, line selector switch and line condition sensing circuit for each of a plurality of telephone lines that are connected to the station and available thereat for individual or concurrent connection to the station set.